Refactor wimpy feature (#2507)

- Make it a common parameter in harness using either '-w', '--wimpy' or
'CL_WIMPY_MODE' environment variable.
- Remove all test specific wimpy variable.

---------

Co-authored-by: Kévin Petit <kpet@free.fr>

test_conformance/half/cl_utils.cpp

@@ -58,7 +58,6 @@ uint32_t gDeviceFrequency = 0;
 uint32_t gComputeDevices = 0;
 size_t gMaxThreadGroupSize = 0;
 size_t gWorkGroupSize = 0;
-bool gWimpyMode = false;
 int gWimpyReductionFactor = 512;
 int gTestDouble = 0;
 bool gHostReset = false;